Lactic acid-glycolic acid copolymers or derivatives thereof such as salts are known as biodegradable polymers and are useful as, for example, materials for sustained release microcapsules for containing physiological active substances. If lactic acid-glycolic acid copolymers used as sustained release formulation materials contain large amounts of lactide and glycolide used as raw materials, these residual materials are hydrolyzed to form acids, which promote the decomposition of the lactic acid-glycolic acid copolymers. Consequently, the sustained release formulations fail to achieve the desired sustained release period. For example, known techniques for producing purified lactic acid-glycolic acid copolymers suited for sustained release formulations are 1) to add water to a solution of a polyhydroxycarboxylic acid in a hydrophilic organic solvent to precipitate a copolymer (Patent Literature 1), 2) to treat a polyester solution in the presence of a precipitation solvent with a high-shear device to produce a copolymer as fine particles (Patent Literature 2), 3) to supply a solution of a polyhydroxycarboxylic acid in an organic solvent and isopropyl alcohol to a device having a kneading mechanism, and to perform operations so as to produce a powdery copolymer (Patent Literature 3), and 4) to treat a lactic acid polymer in the presence of a solvent having a lactide solubility of not less than 4% while performing crushing and stirring with a mixer and thereby to extract residual lactide (Patent Literature 4). However, the purified polymer obtained by the method 1) is a viscous liquid that contains a relatively large amount of the organic solvent remaining in the polymer, and the recovery of the polymer is difficult by filtration and entails a special decantation device. Further, vacuum drying for the purpose of removing the organic solvent and other residual substances encounters difficulties because the polymer is significantly expanded to a large volume by the evaporation of the residual substances such as the organic solvent. The purified polymer obtained during the method 2), 3) or 4) is in the form of viscous liquid or mass even in the case where the polymer is precipitated by reprecipitation or the like. Thus, the direct drying of the polymer does not afford a polymer powder. In order to obtain the polymer as a powder, the method requires a special device such as a high-shear high-speed rotary device or a kneader to break the precipitated polymer by a shear force.
Other known techniques include 5) to dissolve polylactic acid in a solvent followed by cooling to induce phase separation and fractionation (Patent Literature 5), 6) to dissolve polylactic acid into a water-miscible organic solvent and to add an aqueous alkali metal salt solution to the solution (Patent Literature 6), and 7) to hydrolyze a high-molecular weight lactic acid polymer to precipitate a target lactic acid polymer (Patent Literature 7).
These conventional methods have difficulties in realizing industrial and simple production of a purified lactic acid-glycolic acid copolymer or a salt thereof in the form of powder, and are still susceptible to improvements.